And I'm Melissa Block. In London, authorities and the public are grappling with the brutal murder yesterday of a British soldier who was hacked to death on the street. All the evidence suggests this was a terrorist attack by Islamist extremists. Investigators moved quickly today, pursuing leads, making additional arrests and filling in the picture of what happened and who was involved.

Our coverage begins with NPR's Philip Reeves.

PHILIP REEVES, BYLINE: Police are carrying out a huge and complex investigation. At its heart are the two suspects, the two men videoed at the killing site, hands covered in blood, making speeches to anyone who'd listen. They've spend the day in hospital under armed guard after being shot by the police. But did they act alone or with an organization? Today, counterterrorism detectives arrested another two people in London, a man and a woman, both aged 29.

The two are being held on suspicion of conspiracy to murder. The brutality and brazenness of this attack has horrified the British. Prime Minister David Cameron described it as a betrayal of Islam and of Britain's Muslims.

PRIME MINISTER DAVID CAMERON: The people who did this were trying divide us. They should know something like this will only bring us together and make us stronger.

REEVES: The victim was dumped in the street after being hacked to death with knives, a machete and a cleaver. His identity was revealed today. Lee Rigby was a soldier with the British Army's Royal Regiment of Fusiliers. He was 25 and had a 2-year-old son. Rigby did a stint in Afghanistan's Helmand Province, though he lately worked in recruitment.

Tributes to him are pouring in. People have been laying flowers at the south London barracks where he worked, close to the spot where he died. The mood at the scene today was tense. Many questions remain unanswered in this attack. It's emerged the two alleged killers were already known to British security agencies. Their names reportedly showed up in previous British counterterrorism investigations.

A committee of Parliament will now look into all this. It will want to know why the two weren't stopped. Yet, tracking Britain's radical Islamists requires time, people and money. Admiral Alan West, a former government security minister, says more than 2,000 British people are already being monitored.

ADMIRAL ALAN WEST: Most of them are second or third generation in this country so, yes, I mean, it is a real worry. And it's radicalization that happens through a number of causes and that's why we put a lot of effort into trying to stop that radicalization and I think we need to do even more to try and stop that.